WASHINGTON — The two-week-old shutdown has halted one of the federal government’s most important public health activities, the inspections of chemical factories, power plants, oil refineries, water treatment plants, and thousands of other industrial sites for pollution violations.

The Environmental Protection Agency has furloughed most of its roughly 600 pollution inspectors and other workers who monitor compliance with environmental laws. Those scientists, engineers and analysts are responsible for detecting violations that endanger human health, as they did, for example during an August 2018 airborne inspection that found that oil and gas fields in Karnes County, Tex., were leaking illegal levels of chemicals into the atmosphere, in violation of the Clean Air Act.

While the inspection personnel represent a relatively small proportion of the E.P.A.’s total of about 15,000 workers, their absence increases the chances that, either by design or by accident, companies might emit illegal levels of contaminants into the air or water without detection, for weeks on end, according to people familiar with the E.P.A. inspections.

“There are plants that discharge wastewater into streams and rivers, places that store hazardous chemicals in containers that could leak — we show up and test these places to see if they’re meeting pollution laws,” said Garth Connor, a furloughed E.P.A. inspector based in Philadelphia who has been off the job since Monday. “Now there’s nobody out there to check if they’re complying.”

Mr. Connor inspects for air and water pollution and hazardous waste disposal at sites throughout the Mid-Atlantic.

The inspectors “are the cops on the beat,” Cynthia Giles, who headed the E.P.A. enforcement division during the Obama administration, wrote in an email.

She noted that, in 2017, E.P.A. workers performed about 11,700 such inspections, averaging to about 225 inspections per week, according to the agency’s records. The numbers suggest that hundreds of such inspections may have already been canceled this year, with the potential for hundreds more to not take place should the shutdown continue for days or weeks more.

“Those weeks can never be made up,” Ms. Giles wrote. “In addition to the violations not found and the inspections not done, there is also the impact of no inspectors in the field doing unannounced inspections,” she added, asking: “Will that result in more violations because companies know E.P.A. isn’t watching?”

Andrew Wheeler, the acting administrator of the E.P.A., did not respond to an email requesting comment. On Wednesday, President Trump formally nominated Mr. Wheeler, who was confirmed last year as the deputy chief of the agency, to formally take over as the agency head.

When on the job, E.P.A. inspectors regularly cite companies for violations that endanger human health. For example, during an April 2016 inspection at a Firestone rubber plant in Sulphur, La., E.P.A. inspectors discovered that the plant was emitting illegal levels of butadiene, a carcinogen, into the community.

A telephone message left at the plant was not returned.

Some E.P.A. inspections are unannounced. Others take the form of two- and three-week on-site visits.

Still other examinations don’t happen on-site: E.P.A. experts sitting in labs or at computers will review documents detailing a plant’s own reported emissions of pollution or wastewater, checking whether legal limits were met or violated. These activities, too, are on pause during the shutdown.

Inspectors need to read those reports “and say, ‘no, you can’t do that,’” said Eric Schaeffer, who worked at the E.P.A. on enforcement from 1990 to 2002 and now runs the Environmental Integrity Project, an advocacy group. “Then they follow up and go on-site. But none of that is happening.”

Unlike other federal agencies affected by the government shutdown, the E.P.A. continued to operate through the week of Dec. 24, but pollution inspections, along with most of the rest of the work of the agency, had ceased by New Year’s Eve.

Mr. Schaeffer recalled the effect on pollution enforcement of the longest government shutdown in history, which ran from Dec. 16, 1995, to Jan. 6, 1996.

“That was one of the worst years ever at the E.P.A. in terms of numbers of inspections and enforcement,” he said. He added that the damage to the work of pollution inspections didn’t end completely once the government reopened. “Everything was ground to a halt, bogged down. You can’t just restart at 100 miles per hour. You have to reschedule everything.”

Another former E.P.A. official who now lobbies on behalf of industry offered a different view, saying that a shutdown of even a few weeks was unlikely to make much difference in the amount of illegal pollution emitted or detected.

“What you have is a delay,” said the former official, Jeffrey Holmstead, who served in the E.P.A. during both Bush administrations and now works for some of the largest coal companies and electric utilities in the country. “I don’t think it’s true that all of a sudden, because E.P.A.’s inspectors are not there, that most people will take advantage of that,” he said. “There may be a few folks who believe they can get away with more, but I don’t think that’s the biggest issue.”

Among Mr. Holmstead’s clients are several companies that have been cited for violations by the E.P.A., including the electric utility Southern Company, which has had 52 sites with violations over the past five years, including 23 sites with current violations, according to E.P.A.’s enforcement database. An email sent to a Southern Company spokesman requesting comment on the violations was not answered.

Another of Mr. Holmstead’s clients, the electric utility, Ameren, owns 23 sites that have been cited for pollution violations over the past five years. A telephone message left with an Ameren spokeswoman was not returned.

In many years, about 10 to 20 percent of the E.P.A.’s pollution inspections turn up significant violations, according to the agency’s data.

Most operators “really are doing a good job,” said Adam Kushner, a former top legal official at the E.P.A. “But there’s a 1 percent that are bad actors, who will continue to do what they’re going to do, unless inspectors find them. And then there are sites where the operator just may not have identified the problem, and they’re putting bad stuff out into the air without knowing it.”

Angela McFadden, a furloughed E.P.A. environmental engineer who oversees state permits for pollution discharge and has worked as an on-site inspector dealing with clean water violations, said she “always” found violations during her time as an inspector.

For example, she said, in inspecting municipal water systems in rural areas she frequently found that cities and towns over-chlorinated or under-chlorinated their water — not a legal violation, but a potentially harmful situation that is easily corrected when identified by an inspector.

Ms. McFadden recalled a more frightening inspection she once performed in Pennsylvania that found excessive nitrate levels in a municipal water supply. Nitrates can sap oxygen from the blood and, when found in high levels in drinking water, are linked to “blue baby syndrome,” in which infants struggle to deliver enough oxygen to their bodies.

“Right now, E.P.A. is not monitoring any of that,” Ms. McFadden said. “Things are falling through the cracks.”

A picture caption with an earlier version of this article twice misidentified the owner of the E.D. Edwards power station in Bartonville, Ill. The station is owned by Vistra Energy, not Ameren Corp and not Dynegy Inc. (Ameren sold the plant to Dynegy in 2013. Dynegy folded into Vistra in 2018.) The picture has been removed.

Correction:

An earlier version of this article described incorrectly Angela McFadden's role at the Environmental Protection Agency. She is an environmental engineer who works for an agency program that oversees state-issued permits for the discharge of pollution in rivers and streams. Though she has worked as an on-site inspector dealing with clean water violations, she is not an inspector of water sites.

The article also misquoted Ms. McFadden in her comments about her work at the E.P.A. While Ms. McFadden said, “I always find violations,” she did not go on to say, “even if it’s not things that are illegal.”

The article also suggested that problems with chlorine levels in water are especially common in rural West Virginia. That is not the case.

Coral Davenport covers energy and environmental policy, with a focus on climate change, from the Washington bureau. She joined The Times in 2013 and previously worked at Congressional Quarterly, Politico and National Journal. @CoralMDavenport•Facebook

A version of this article appears in print on , on Page A13 of the New York edition with the headline: Now, ‘Nobody Out There’ Keeping Eye on Pollution. Order Reprints | Today’s Paper | Subscribe